Variable resistance fuel level senders are known in the art for indicating the level of liquid fuel in a vehicle fuel tank. Such fuel senders typically comprise a housing or body mounted in or on the interior of the fuel tank, a resistor card mounted on the housing, a float arm assembly pivotally connected to the housing, and a conductive wiper connected to and operated by the float arm. The conductive wiper has a sliding contact which moves along the resistive path of the card, and a ground contact to a ground terminal. A positive terminal on the housing supplies electric current to the card with suitable connections from a remote power source.
The resistive path on the card is often arcuate, comprising a plurality of conductive pads with varying resistance along the length of the arc. The float arm pivots on an axis approximately coincident with the center of the arc. The float arm assembly includes a bent wire float arm with a float mounted at its free end and a pivot pin extending through a hole in the housing. A plastic support arm or carrier is connected to the float arm and rotates with the float arm, transferring float arm rotation to the conductive wiper, and provides stop surfaces which interact with stops on the housing to define the limit of travel of the float arm and wiper.
Such an assembly is found in the Stewart-Warner Corporation fuel sender used in the Ford Motor Co. 1992 Saturn automobile. Other fuel senders having similar structure are shown in U.S. Pat. Nos. 4,873,865 and 4,924,704 to Gaston, and 4,700,170 to Weaver. An example of a typical resistor card is disclosed in U.S. Pat. No. 4,931,764 to Gaston.
An omnipresent concern with these types of fuel senders is the need for uniformity, stability and consistency in the fit and operation of the moving parts over the lifetime of the unit. Metal-to-metal wear, plastic creep and thermal expansion caused by temperature changes in the fuel tank environment, fuel additive effects on electrical contact surfaces and plastics, relaxation of spring tension, unbalanced moment forces, and inherently weak or unstable mounting and support arrangements can all affect the reliability and repeatability of the readings from the fuel sender unit. Some of the areas particularly critical to reliable and consistent functioning are the sliding wiper interfaces, the spring rates and contact forces of the conductive wiper, the pivot support and bearing surfaces of the float arm assembly, and the different expansion rates of metal-to-plastic connections. Reducing the amount of variation, imbalance and loose fit in these areas of the fuel sender greatly increases the unit's reliability and calibration accuracy. None of the prior art to date has adequately and systematically addressed the above-listed factors in a comprehensive manner.